(CBS) The FDA announced today that not all of the tainted pet food has been pulled from store shelves. And at a congressional hearing, an entirely new concern was raised: How close did contaminated ingredients come to getting into food for humans?

CBS News has learned that the tainted wheat gluten used in pet food was human grade — meaning nothing but luck kept it from being used in the food people eat, too, CBS News correspondent Sharyl Attkisson reports.

Wheat gluten is added to foods like bread, pasta and rice. While the public was focused on the danger to their pets, sources tell CBS News that the FDA had tracked at least one suspect batch of wheat gluten into the human food supply, quietly quarantined some products and notified the Centers For Disease Control and Prevention to watch for new patients admitted to hospitals with renal or kidney failure.

"We didn't know at the time whether or not wheat gluten had made it into the human food supply," said Dr. Stephen Sundlof, director of the FDA's Center for Veterinary Medicine. "We asked CDC to put a special emphasis on looking at increased incidents of renal failure in people."

But there were no spikes in illnesses, and the human food ultimately tested clean. The FDA tried to comfort Congress today, saying there's "no evidence" any bad gluten got into human food — thought the agency still doesn't know where it all went.

"What disturbs me about this incident is that it confirms yet again that pet food as well as human food is at risk," said Sen. Richard Durbin, D-Ill.

Menu Foods, the food supplier that seems to have bought most of the problem ingredient, wouldn't testify today. The lobby group that came in its place was left to explain why Menu supposedly knew that tests were making animals sick on Feb. 20 but didn't tell the FDA until three weeks later.

"Let's get the record straight: Menu waited more than three weeks after finding out that the dogs wouldn't eat their food and were getting sick. They waited three weeks!" Durbin said.

"I don't have the facts on Menu, senator,' Duane Ekedahl of the Pet Food Institute testified.

"I think before you came to the hearing you would have the facts!" Durbin replied.

The FDA has gotten more than 19,000 phone calls, but today said it has "no good numbers" and that there's no way to tell how many animals have gotten sick.

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